1. Field of the Invention
The present invention relates to systems and methods for determining ionic or neutral concentrations within a medium using a passive remote sensing technique.
2. Description of the Related Art
FIG. 1 illustrates different atmospheric layers of the Earth's atmosphere.
As shown in the figure, up to about 14 kilometers (km) directly above the Earth 100 is the troposphere 102. After the troposphere 102 is the tropopause 104, which is only about 4 km thick. The stratosphere 106 is directly above the tropopause 104. Within the stratosphere 106 is the ozone layer 108. Next is the mesosphere 110, which is about 40 km thick. Finally, the ionosphere 112, directly above the mesosphere 110, is hundreds of kilometers thick. The neutral component of the atmosphere above the mesosphere is referred to as the thermosphere. Together, the ionosphere, thermosphere and mesosphere are commonly referred to as the upper atmosphere.
With respect to telecommunications, the ionosphere is particularly important.
The ionosphere is the ionized part of the atmosphere produced primarily by the absorption of solar radiation. The principle component of the upper part of the ionosphere is singly ionized atomic oxygen (O+). The ionosphere has practical importance because, among other functions, it influences radio wave propagation to distant places on the Earth. The influence extends across a wide range of radio frequency bands, well above the high frequency band, considered to be 3-30 megahertz (MHz.) The effects include impacts on radio transmissions in all bands, e.g., amplitude modulation (AM), frequency modulation (FM), shortwave, etc., and radars (including over the horizon radars).
Satellite-borne remote sensing of the ionosphere observe emissions by atomic ions (singly ionized atomic oxygen (O+)) and the neutral components of the upper atmosphere such as atomic oxygen (O), molecular oxygen (O2), molecular nitrogen (N2), nitric oxide (NO), ozone (O3), helium (He), hydrogen (H). FIGS. 2A-2C illustrate such a system.
FIGS. 2A-C illustrate a conventional satellite-based method of measuring ionic concentrations within the Earth's ionosphere. FIG. 2A illustrates measurements taken at a first time t1. FIG. 2B illustrate measurements taken at a second time t2. FIG. 2C illustrate locations of calculated ionic concentrations using the measurements at times t1 and t2.
As shown in FIG. 2A, a satellite 202 and a satellite 204 are located in space 206 above the Earth's ionosphere 208, which is illustrated as having a lower boundary 210 and an upper boundary 212.
At time t1, satellite 202 measures the total emissions of a particular ion along a line-of-sight (LOS) 214, whereas satellite 204 measures the total emissions of the ion along a LOS 216, a LOS 218 and a LOS 220.
In the conventional method of FIG. 2A, satellite 202 is able to detect a total of emissions by a particular ion, for example, atomic oxygen ions (O+), within ionosphere 208 along LOS 214.
What is more valuable for radio wave communications is an altitude profile of the amount of the particular ion, in this example atomic oxygen ions (O+) at each altitude z, or [O+](z). In other words, in addition to the total amount of emission along LOS 214, an altitude function [O+](z) of the O+ number density along LOS 214 would be valuable. A mapping of such altitude functions along the path of a vehicle traveling above the earth would greatly enable high frequency (HF) communication systems to compensate for negative impacts of our otherwise imperfect knowledge of the altitude profile of atomic oxygen ions on HF and radio frequency signals.
The altitude function of the particular ion is formulated by tomographic retrieval. The mathematical basis for tomographic retrieval is applied to obtain cross-sectional images and is based on the notion that a projection of an object at a given angle θ is made up of a set of line integrals. In ionospheric observations, the line integral represents the total emissions along a line-of-sight (LOS) through the ionosphere. It is known that if there are an infinite number of one-dimensional projections of an object taken at an infinite number of angles, the original object can be reconstructed. To accomplish this, a filtered back projection algorithm is used. Accordingly, to find the altitude function of the particular ion, the individual ion concentrations along LOS 214 via satellite 204 are first determined. For example, satellite 204 is able to detect a total of emissions by the same ion as satellite 202, in this example O+, within ionosphere 208 along LOSs 216, 218 and 220.
Here, LOSs 214, 216, 218 and 220 are in the same plane, i.e. the plane of the figure, such that: LOS 214 intersects with LOS 216 at location 222; LOS 214 intersects with LOS 218 at location 224; and LOS 214 intersects with LOS 220 at location 226. Clearly, satellite 204 may detect total emissions within ionosphere 208 along more LOSs, however, for purposes of discussion, a sampling of LOSs 216, 218 and 220 are provided.
In order to tomographically retrieve the ion altitude function of the entire plane of ionosphere 208 (a ribbon in the plane of the figure), satellites 202 and 204 must scan additional areas. This will be described with reference to FIG. 2B.
As shown in FIG. 2B, satellite 202 and satellite 204 are located at new locations in space 206 above ionosphere 208.
At time t2, satellite 202 measures the total emissions of the particular ion along a LOS 228, whereas satellite 204 measures the total emissions of the ion along a LOS 230, a LOS 232 and a LOS 234.
Here, LOSs 228, 230, 232 and 234 are in the same plane, i.e. the plane of the figure, such that: LOS 228 intersects with LOS 230 at location 236; LOS 228 intersects with LOS 232 at location 238; and LOS 228 intersects with LOS 234 at location 240. Clearly, satellite 204 may detect total emissions within ionosphere 208 along more LOSs, however, for purposes of discussion, a sampling of LOSs 230, 232 and 234 are provided.
The detected total emissions along a LOS includes emission contributions from ions within the LOS in addition to emission contributions from neighboring ions, taking into account secondary emission issues related to resonance, fluorescence, etc. This will be described with reference to FIG. 2C.
As shown in FIG. 2C, locations 222, 224 and 226 are determined from the intersecting LOSs of FIG. 2A, whereas locations 236, 238 and 240 are determined from the intersecting LOSs of FIG. 2B. Here the emission detected by satellite 202 (and 204 for that matter) at location 222 includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at locations 224, 226, 236, 238 and 240. Similarly, emission detected by satellite 202 (and 204 for that matter) at location 236 includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at locations 222, 224, 226, 238 and 240.
As satellites 202 and 204 scan the remainder of the plane within ionosphere 208, an array of emission values will be determined. If more LOSs are used, then more emission values will be determined, i.e., the larger the array. Once the emission values are determined, any known method may be used to determine the ion altitude function for the entire plane of ionosphere 208.
Once the ion altitude function for the entire plane of ionosphere 208 is known, it may be taken into account when transmitting/receiving signals therethrough.
All conventional systems for measuring ionic concentrations within the Earth's ionosphere are not satellite-based.
FIGS. 3A-C illustrate a conventional system of ground-based detectors used to deduce the properties of the ionosphere. The geometry illustrated in FIG. 3A-C has been applied to radio-based remote sensing of ionospheric properties. FIG. 3A illustrates measurements taken at a first time t1. FIG. 3B illustrate measurements taken at a second time t2. FIG. 3C illustrates locations of calculated ionic concentrations using the measurements at times t1 and t2.
As shown in FIG. 3A, a ground-based detector 302 and a ground-based detector 304 are located below ionosphere 208. The system of FIG. 3A operates in a similar manner to that of the system of FIG. 2A. However, in the system of FIG. 3A, the LOSs are directed from the Earth to ionosphere 208.
At time t1, ground-based detector 302 measures the total emissions of a particular ion along a LOS 314, whereas ground-based detector 304 measures the total emissions of the ion along a LOS 316, a LOS 318 and a LOS 320.
The altitude function of the particular ion is formulated by initially finding individual ion concentrations along LOS 314 via ground-based detector 302. Ground-based detector 304 is able to detect a total of emissions by the same ion ground-based detector 302, in this example O+, within ionosphere 208 along LOSs 316, 318 and 320.
Here, LOSs 314, 316, 318 and 320 are in the same plane, i.e. the plane of the figure, such that: LOS 314 intersects with LOS 316 at location 322; LOS 314 intersects with LOS 318 at location 324; and LOS 314 intersects with LOS 320 at location 326. Clearly, ground-based detector 304 may detect total emissions within ionosphere 208 along more LOSs, however, for purposes of discussion, a sampling of LOSs 316, 318 and 320 are provided.
As shown in FIG. 3B, ground-based detector 302 and ground-based detector 304 are located in the same positions as described above with reference to FIG. 3A. However, in this situation, ground-based detector 302 is detecting along a new LOS and ground-based detector 304 is detecting along new LOSs.
At time t2, ground-based detector 302 measures the total emissions of the particular ion along a LOS 328, whereas ground-based detector 304 measures the total emissions of the ion along LOS 330, a LOS 332 and a LOS 334.
Here, LOSs 328, 330, 332 and 334 are in the same plane, i.e. the plane of the figure, such that: LOS 328 intersects with LOS 330 at location 336; LOS 328 intersects with LOS 332 at location 338; and LOS 328 intersects with LOS 334 at location 340. Clearly, ground-based detector 304 may detect total emissions within ionosphere 208 along more LOSs, however, for purposes of discussion, a sampling of LOSs 330, 332 and 334 are provided.
As mentioned previously, the detected total emission along a LOS includes emission contributions from ions within the LOS in addition to emission contributions from neighboring ions, taking into account secondary emission issues related to resonance, fluorescence, etc. This will be further described with reference to FIG. 3C.
As shown in FIG. 3C, locations 322, 324 and 326 are determined from the intersecting LOSs of FIG. 3A, whereas locations 336, 338 and 340 are determined from the intersecting LOSs of FIG. 3B. Here the emission detected by ground-based detector 302 (and 304 for that matter) at location 322 includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at location 324, 326, 336, 338 and 340. Similarly, emission detected by ground-based detector 302 (and 304 for that matter) at location 336 includes secondary emissions related to resonance, fluorescence, etc., as contributed by the ions at location 322, 324, 326, 338 and 340.
As ground-based detectors 302 and 304 scan the remainder of the plane within ionosphere 208, an array of emission values will be determined. If more LOSs are used, then more emission values will be determined, i.e., the larger the array. Once the emission values are determined, any known method may be used to determine the ion altitude function for the entire plane of ionosphere 208.
Once the ion altitude function for the entire plane of ionosphere 208 is known, it may be taken into account when transmitting/receiving signals therethrough.
Of the conventional systems discussed above, they are limited to determining the ion altitude function from above a medium or from below a medium.
What is needed is system and method for determining the ion altitude function of a medium from within the medium.